A building block of many electronic devices such as diodes, transistors, and lasers is semiconductor material that can be grown over a substrate. The semiconductor material is fabricated by growing an epitaxial layer of the semiconductor material upon a substrate. The substrate material may be, and frequently is, of a different composition and lattice parameter than the material used to grow the epitaxial layer. For example, an epitaxial layer of gallium nitride (GaN) may be grown upon a sapphire substrate.
Various light emitting structures may be fabricated from the semiconductor material. To complete these structures, it is often desirable to detach and transfer the epitaxial layer from the original substrate to another substrate. For example, when fabricating an edge-emitting laser, it is necessary to cleave the epitaxial layer to obtain a reflecting surface. Cleaving is a process by which an edge of a wafer, including a substrate and an epitaxial layer, is nicked, or scored, and the wafer snapped to expose a clean, smooth cross-section of the epitaxial material along the lattice facet. A parallel pair of facets are used as reflective mirrors in an edge-emitting semiconductor laser.
Cleaving, however, is difficult to perform while the GaN epitaxial layer is attached to a sapphire (Al.sub.2 O.sub.3) substrate. Therefore, when a GaN epitaxial layer is grown upon sapphire, it is desirable to separate the GaN from the sapphire substrate. This is so because the sapphire substrate does not cleave easily, and also because the sapphire and the GaN epitaxial material have different lattice constants, resulting in lattice planes that do not accurately align. This prevents obtaining a clean, parallel facet of the epitaxial layer by cleaving if the epitaxial layer and the sapphire substrate are attached to each other. Therefore, for edge-emitting lasers it is desirable to detach and transfer the epitaxial layer from the sapphire substrate to a different substrate, such as silicon (Si), that is more easily cleaved.
A vertical cavity surface emitting laser (VCSEL) generally includes a region of multiple quantum well layers composed of very thin alternating layers of GaN and AlGaN around which are formed distributed Bragg reflectors.
In conventional VCSELs, at least the DBR between the substrate and the quantum-well layers is formed from alternating GaN and AlGaN layers so that the quantum well layers can be grown on crystalline material with about the correct lattice parameters. However, crystalline GaN/AlGaN DBRs are difficult to make with sufficient reflectivity. Dielectric DBRs composed of alternating layers of two dielectric materials such as silicon dioxide (SiO2) and hafnium oxide (HfOx) are easier to make with the required reflectivity, but quantum well layers cannot be grown on top of these dielectric layers.
It would be desirable to be able to grow a partial VCSEL structure, including a first cladding layer, a set of quantum well layers, a second cladding layer and a first dielectric DBR, on one substrate and transfer it to another substrate. This places the second cladding layer and the first DBR in contact with the new substrate and exposes the first cladding layer, over which could then be grown a second dielectric DBR.
In the past, epitaxial layers have been separated from sapphire substrates by using a laser to melt the epitaxial layer at its interface with the substrate on which it is grown. In the case of a GaN epitaxial layer grown over a sapphire substrate, the output of an ultra-violet laser is directed through the sapphire substrate and melts a thin layer of the GaN near the GaN/sapphire interface, thus enabling the GaN epitaxial layer to be separated from the sapphire substrate.
A drawback with using a laser to separate an epitaxial layer from the substrate is that due to the limitation of the laser spot size, only a small portion of the epitaxial layer may be separated at a time.
Another drawback with using a laser to separate the epitaxial layer from the substrate is that it is difficult to control the penetration depth of the laser light. This may result in a portion of the GaN epitaxial layer being rendered potentially unusable due to surface roughening and thermal shock within the material. The penetration depth is difficult to control because the heat dissipated at the GaN/sapphire interface cannot be precisely controlled.
Thus, an unaddressed need exists in the industry for a more controllable, practical method for detaching an epitaxial layer from a substrate and transferring the epitaxial layer to another substrate.